A conventional flexible magnetic disc unit is not provided with alternate processing of defective sectors, since the capacity of the medium itself is small and the medium can be exchanged for another. Instead, a defect mark is placed on a defective sector and that area is not used, thereby making it possible to avoid accessing the defective sector.
Furthermore, in a fixed magnetic disc drive apparatus, when a host unit designates an objective track number, the objective track is once accessed to read alternate information which has been written in an ID field of the objective sector. Thereby, alternate processing is performed either by accessing to a spare sector located in the same cylinder, after waiting for a rotation or by having a second access to an alternate area provided in a separate area.
In the above-described conventional flexible magnetic disc unit, however, an objective replaced sector is to accessed only after first accessing the defective sector to read its ID field and then accessing a spare sector in the same cylinder or an alternate area provided in a separate cylinder. This results in problems as follows.
When an alternate sector for a dective sector is designated, the alternate sector is accessed after accessing to the sector. Thus, two or more operation are required, thus the access efficiency to the magnetic disc drive apparatus is deteriorated.
When alternate information is required additionally in a medium which has been formatted once, it is normally required to rewrite alternate information in the ID field. Since it is impossible to know the position of the ID field accurately in a flexible magnetic disc unit in which a medium is to be changed without physical positional information (sector pulse) showing the ID field on the medium, it is impossible to rewrite ID information in flexible magnetic discs.
Moreover, when a defect is generated in the ID information loaded in a sector on the medium, it becomes impossible to load alternate information of the sector and to perform alternate processing.